The present invention relates to a sewing machine having guide means and ironing means for folding back and ironing stitched margins of upper and lower pieces of fabric.
There are known sewing machines including guide means for folding stitched margins of upper and lower fabric pieces which have been sewn by the sewing machine and fed by a fabric feeder, and an ultrasonic heating means for heating and ironing the folded sewn margins. When the upper and lower fabric pieces are fed into an area where they will be heated by the ultrasonic heating means, and are ironed thereby, the electric power supplied to the ultrasonic heating means is controlled so as to be of a constant level.
Since the electric power supplied to the ultrasonic heating means for pressing or creasing the stitched margins is of a constant level, when the stitched margins have an increased thickness due to a stepping region in a sewing process, such a stepped region cannot be ironed and creased effectively.
Further, the ultrasonic heating means generally includes an ultrasonic horn and an anvil which jointly provide a heating region where the upper and lower fabric pieces are supplied and ironed with heat. The distance between the anvil and the ultrasonic horn is adjusted by a manual adjusting means such as a screw in accordance with a total thickness of the upper and lower fabrics.
Since the distance between the anvil and the ultrasonic horn is manually set to a certain value, when the margins of the fabric pieces have a stepped area or the thickness of the fabric varies in a sewing process because of a different kind of fabric used, the pressure applied to the margins of the fabric pieces by the ultrasonic horn also varies, and hence the ironing effect of ultrasonic horn on the fabric margins to crease them is not rendered constant. When the thickness of the fabric varies greatly, the fabric pieces are not smoothly fed through a space defined between the ultrasonic horn and the anvil, but tend to remain therebetween. The fabric pieces thus caught between the ultrasonic horn and the anvil are overheated and burned, thus losing the commercial value of the finished product.
Related technique is disclosed in a commonly assigned U.S. Pat. No. 4,823,713 entitled "Sewing Machine with an Ultrasonic Heater for Folding Back Sewn Edges". In this patent, an ultrasonic horn is employed for heating the folded margins of the upper and lower fabric pieces. With the sewing speed being changed, i.e., with the fabric feeding speed being changed dependent on the change in fabric feed pitch, ultrasonic intensity is varied so as to provide a constant heating energy to the fabrics in spite of the irregular fabric feeding speed. The ultrasonic heating is being made only when the fabrics are moved. In this patent if the total thickness of the upper and lower fabrics is changed, the fabrics cannot undergo uniform heating or ironing. Therefore, relevant inherent drawbacks are still involved in the technique described in the U.S. patent.
Related techniques are also disclosed in other U.S. Pat. Nos. 4,495,879, 4,098,205, 3,976,020, 3,587,496 and 3,246,616, those in which a heating plate is used as a heating means. The heating plate performs thermal conduction directly to the fabrics. However, in such thermal conduction type, it would be rather difficult to abruptly control the heating energy. Further, if the fabrics provide a stepped portion, it would be rather difficult to provide a proper surface contact of the heating plate to the stepped portion.